Such surface emitting semiconductor lasers are also known by the terms disc laser, VECSEL (Vertical External Cavity Surface Emitting Laser) or OPSL (Optically Pumped Semiconductor Laser) and are described for example in the published U.S. patent application Ser. No. 2002/0001328, U.S. patent application Ser. No. 10/53,364 and U.S. Pat. No. 6,243,407, the content of all of which is hereby incorporated by reference.
Surface emitting semiconductor lasers that are used in particular in the infrared spectral region emit at high pump powers in a spectrally broadband fashion in a wavelength range that may be a plurality of nanometers wide. Therefore, it is often necessary to stabilize the emission wavelength and to control the spectral width of the emission. This is the case particularly when a resonator-internal frequency doubling is provided.
With regard to a high quality factor of the resonator and a broadband emission, dye lasers or titanium-sapphire lasers represent systems comparable to the disc laser. In the case of these lasers, so-called etalons and bifringent filters are used for controlling the spectral properties of the emission. Etalons are based on the principle of multiple reflection at plane-parallel laminae and enable the wavelength to be tuned in a limited range by variation of the angle of incidence. Such etalons fundamentally require a largely parallel beam course.
Bifringent filters lead to wavelength- and polarization-dependent losses and stabilize the laser as a result at a preferred wavelength. They are suitable particularly for tunable lasers since the wavelength of the laser can be set in a wide range by means of azimuthal rotation of the filter without influencing the other resonator conditions.
Both etalons and bifringent filters are suitable for comparatively low-loss resonator-internal wavelength selection. However, they require a resonator design with little beam divergence and impose high requirements on the alignment accuracy of the angle of incidence or of the azimuthal angle.
The use of a bifringent filter for wavelength selection in a disc laser is disclosed in U.S. Pat. No. 6,167,068. This document also describes a resonator-internal wavelength conversion of the surface emitting semiconductor laser.